The surface area of roots is estimated to be 100 times or more that of above-ground parts, and a huge number of root hairs absorb water, inorganic nutrients, and the like. In recent years, the importance of the root system as “The Hidden Half” has been realized again, and the application thereof to a grafting technique has been expected to be developed. The grafting technique is a cultivation method in which, in general, a root part serving as a rootstock and an above-ground part serving as a scion are plant bodies of related species with different genomes, and the excellent abilities of both parts are made to work collaboratively. There is a record that grafting has been carried out for cultivating citrus fruits in China for about 4000 years. The original purpose of grafting is to maintain excellent individuals or impart an early flowering property, however, the existence of a rootstock species having a more excellent root system was found, and a grafting process preferentially using such rootstock species has been adopted. The merit of grafting is large in perennial woody plants, however, the reason why grafting is also adopted in annual crops such as tomato, eggplant, and cucumber is because there is an advantage of leading to the improvement of yield by using a rootstock having a higher ability.
However, the grafting technique has not been at all adopted in crops such as soybean and plants of the Poaceae family including rice, wheat, corn, etc. The reason for this is because grafting of young plants of plants of the Poaceae family is difficult, or soybean or the like is originally a plant, for which grafting can be carried out using a seedling thereof (see, for example, Nonpatent Document 1), however, such a plant is cultivated not by transplanting young plants in fields, but by directly sowing seeds in fields. Therefore, if a grafting technique effective for such crops is developed, a rootstock species having an excellent root system can be immediately practically used, so that a significant improvement of the productivity can be realized, however, such a proposal has not yet been presented.